Having some SWR trouble with the MFJ 1890 10 meter moxon

[Alan Blackmon]

I ordered the MFJ 10 meter moxon beam from HRO. I have it assembled and I cannot get a better SWR reading than 1.4 at 28.5 MHz. At 28.0 MHz it is 2.2. At 29.5 MHz it is 2.5. The measurements were taken at 12 and 22 feet above ground. No change of SWR results at either height. If I lay the antenna down and it is pointing straight up from the ground I get 1.2 SWR. So I feel I have done something wrong in putting this antenna together.

I have home built two Moxons. One for 6 meters and one for 2 meters. Both have a very nice tune at <1.6 at each end of the band and 1.2 at the center. Both the home brew ones were PVC frame with wire attached. So I think I know how to build and tune a Moxon. The commercial MFJ Moxon is all metal. Reviewers say they archived a much better SWR result than I have. So I think I may have made an error somewhere.

I know I have not included a ton of information. I will just say I have followed the instructions (which were not all that clear to my 71 year old brain) and I am not having the success I need to use this antenna. After home building two Moxons I used my experience to get the MFJ put together. Is there a common mistake that occurs in assembly that I may have repeated based on experience others may have had themselves and were able to correct? I am stalling contacting MFJ due to the holiday season. It is Dec 27 and I doubt I will get a response for awhile from them. So I am reaching out to this forum.

Thanks for your input. 73

 

[Crawdad]

Could possibly be the distance between the element ends? Maybe? Something to play with anyway.
Might be worth modeling if possible.

73

 

[Alan Blackmon]

Could possibly be the distance between the element ends? Maybe? Something to play with anyway.
Might be worth modeling if possible.

73

I did move the ends closer together. The gap was listed as 6.25 inches. I moved the gap to 4.5 inches with no real change in results. I am wondering if the feed point is wrong. I put the feed point at 3/4 inch from insulators. The driven element ends are separated by an inch but seem to be fed some distance apart. I have a suspicion I have done the feed point wrong.

 

[w9cll]

Take it apart and start over

 

[BJ radionut]

Alan: Did you use the "Matching Stub" (Balun) section supplied, or did you hook your feedline directly at the feed point???
Check every length/dimension again, make sure everything is as it is stated in the assembly. Then test again at easily reached height above ground.
Remember the mast placement on the boom will have some effect to the tuning.
So once tuned this must remain constant.
Good Luck
HNY
All the Best
Gary/W9FNB

 

[wavrider]

Alan keep plugging away at it, you will get it tuned and in the air making contacts soon.
its going to be something simple

 

[Alan Blackmon]

I have connected both the supplied stub and direct connection. No difference. I will keep working at it.

I have a two foot section of wood dowel above my metal mast. I will remove the wood section and try again.

Thanks for your input. It is appreciated.

AB7XX

 

[Alan Blackmon]

Take it apart and start over

I am thinking of doing exactly that. Also removing a wood section on the mast.

AB7XX

 

[Alan Blackmon]

I have disassembled the antenna. It is snowing here in Boise and very cold outside. I won't be getting back to this project until it warms up some. My plan is to wash down the aluminum pieces. When handling it seems I get a residue on my hands. Not sure what this is but I am going to wash it off then reassemble. I will put a bit of anti seize on the pieces where they fit into the next section of the antenna. This is the same anti-seize I have on my 6BTV.

 

[HomerBB]

Here are the dimensions I used for making my all metal tubing 11m Moxon.

As for the placement of your feedpoint wires, that shouldn't matter except for in one small detail. If your peeling back a coax pigtail and connecting to the feedpoint the length of the peeled coax ends will add length to the elements overall length.
It seems to me that your antenna may be too long for 10m. The SWR is getting better going lower on the band which indicates it may be tuned down in the 11m band.
If it were me, I would use a Moxon calculator to see what it suggests for antenna dimensions and compare them to the MFJ dimensions.
Homer

 

[wavrider]

I have disassembled the antenna. It is snowing here in Boise and very cold outside. I won't be getting back to this project until it warms up some. My plan is to wash down the aluminum pieces. When handling it seems I get a residue on my hands. Not sure what this is but I am going to wash it off then reassemble. I will put a bit of anti seize on the pieces where they fit into the next section of the antenna. This is the same anti-seize I have on my 6BTV.

small antenna,,build it in the living room XYL will not mind

 

[Alan Blackmon]

So I did the disassembly the reassembled the antenna this week. I did a wash down of the elements. I put it back together to factory measurements and still had a higher SWR reading than I would like. Soooo I did something outrageous.

I recalibrated my nano VNA. I had not done it in awhile so thought I have nothing to lose. Well folks I had everything to gain. My SWR dropped like a rock. I now have 1:1.6 at 29.3 MHz. Band ends go up of course but I made huge progress. 28 MHz was way up over what I'd like to see.

When I get a warmer day that allows me to touch the metal mast to raise and lower the antenna for adjustments I will lengthen the elements and shift the resonance down towards 28.4 ish and see what 28 MHz offers?

Lesson learned. Calibrate VNA more often.

 

[Alan Blackmon]

As for the placement of your feedpoint wires, that shouldn't matter except for in one small detail. If your peeling back a coax pigtail and connecting to the feedpoint the length of the peeled coax ends will add length to the elements overall length.

Homer[/QUOTE]

Thanks Homer for your input. You gave me a brain jog. I had a little accident with the MFJ supplied feed point wire. I made another one myself. It is shorter than the MFJ supplied one. Being shorter the resonance went higher up in the 10 meter band. Hmmmm, I thought. I should build or buy a new one. If I build it make the wires longer like on the original and I will get the resonance more towards 28.4 ish than the 29.3 it is now. Then I will likely not have to do any element adjustments.

I am betting that the coax feed point wires for the 6 10 and 15 meter moxon will be the same. I am ordering the 6 meter Moxon now that I have this one figured out. I did suspect that it was user error from the beginning. Well yes it was! I can just copy the new feed point coax assembly and I'll likely be good to go.

Your input is greatly appreciated.

 

[Alan Blackmon]

In the spirit of letting you know how it went I am back again with another update. I have rebuilt my feed cable. I am getting around a 1.5:1 SWR with my nano VNA and RigExpert. Antenna is at 23 ft high on temporary mast system. I can use 28 to 28.9 easily with that SWR result. Above that I have a higher SWR reading.

I made a contact from Boise to NJ. He gave me a 5/9 plus report with me at 100watts. He was coming in to me the same. So that is a pleasing outcome with this antenna.

Now despite what my RigExpert reports my radio says my SWR is 1.3:1 so the radio is very pleased to use the Moxon. I am going to call it a success with this antenna. Further testing will need to be done to determine F/B on it.

Thanks to all.

 

[Flatop]

Mate, never rely on a Nano VNA, especially if you don't re-calibrate them regularly.
I would have gone straight to a trusted SWR meter, even an onboard one,to save the wild goose chase. Trust me, I've been there before.